1. Field of the Invention
The present invention relates to a wide-angle lens system, as well as to a manufacturing method therefor and to optical devices in which the wide-angle lens system is used, and more particularly to a wide-angle lens system that includes a first lens group that is located on the object side and has an overall negative refractive power and a second lens group that is located on the image side of the first lens group and has an overall positive refractive power, as well as to a technology pertaining thereto.
2. Description of the Related Art
Wide-angle lens systems that have a large imaging area are used not only for a special purpose, such as for use as a fish-eye lens, but are widely used in the imaging systems of such devices as document display devices that obtain images of documents from a relatively close distance. The required specifications for such wide-angle lens systems include not only a large imaging area, but also adequate corner illumination, small distortion, a compact configuration, and a small number of easily manufactured component lenses. Various types of wide-angle lens systems that meet these criteria and include a zoom function as well have been proposed (see, for example, Japanese Patent Laid-Open No. H09-21946).
A widely used type of wide-angle lens system of the conventional art is a so-called retrofocus type lens system comprising a combination of a first lens group having a negative refractive power and a second lens group having a positive refractive power, and various combinations of lenses have been proposed (see, for example, Japanese Patent Laid-Open No. H11-30743). This type of lens system is characterized in that it has a long back focal length and a relatively high amount of corner illumination.
However, the retrofocus type wide-angle lens system has the problem of high negative distortion, and depending on the lens selection and design, it also entails the problems of significant coma aberration and astigmatism. In particular, as the field angle increases, aberration increases and correction becomes necessary. FIG. 13 is an explanatory drawing showing one example of a retrofocus lens system used in the conventional art. FIG. 14 shows the characteristics of this lens system with regard to spherical aberration, astigmatism and distortion. As shown in the figure, aberration, particularly distortion, is extremely high in this type of lens system, and a substantial amount of distortion occurs in the captured image in the periphery of the imaging area. The parameters for the various types of aberration shown in FIG. 14 are explained in detail in comparison with the characteristics of the wide-angle lens system of an embodiment of the present invention.
While these types of aberration can be eliminated to some extent using such methods as increasing the number of lenses or making a lens surface aspherical, if the number of lenses is increased, the construction becomes complex, making the lens system more difficult and costly to manufacture. Where an aspherical lens is to be used in order to efficiently correct for distortion, because the lens configuration entails significant variation in lens thickness from the optical axis to the periphery of the lens, it is difficult to manufacture the lens with precision.
An object of the present invention is to resolve these problems and to provide a wide-angle lens system that offers small distortion yet does not increase the number of lenses and uses an aspherical lens that is easy to manufacture.